Despite previous warnings that adolescent girls who receive vaccines for human papillomavirus (HPV) would be more sexually active, a new study has found those concerns to be grossly unwarranted.

The new study, conducted by Robert Bednarczyk PhD, of Kaiser Permanente Center for Health Research-Southeast in Atlanta, and his colleagues, have found there is no difference in the sexual behaviors of adolescent girls who have received the HPV vaccine and their peers who have not been given the shot.

The vaccine, given to ultimately prevent cervical cancer, was believed by some opponents to encourage young girls to become more sexually active or to engage in riskier sex than they otherwise would without the shot. But this is not the case, according to the study, published online today in the journal Pediatrics.

For the study, researchers didn´t ask girls about having sex, but instead looked at “markers” of sexual activity after HPV vaccination. Specifically, they examined up to three years of records on whether girls had sought birth control advice; tests for sexually transmitted diseases or pregnancy; or had become pregnant.

The HPV shot has been recommended by the US Centers for Disease Control and Prevention (CDC) since 2006 for girls beginning at age 11. HPV is transmitted through sexual activity and causes genital warts and can lead to cervical, penile, vaginal, and head and neck cancers. The vaccine is also recommended for older girls who did not receive the vaccine when they were younger, and also for males at age 11 and older.

Despite recommendations, the vaccine has been slow to catch on. In the 5 years since becoming available to adolescent girls, only half have received even one dose of Gardasil. Since its introduction, the vaccine has been associated with increased sexual activity in teens. However, the Kaiser study, with a partnership from Emory University, of which Bednarczyk is also a member, has shown there is little evidence to warrant these claims.

“Our study found a very similar rate of testing, diagnosis and counseling among girls who received the vaccine and girls who did not,” said Bednarczyk. “We saw no increase in pregnancies, sexually transmitted infections or birth control counseling — all of which suggest the HPV vaccine does not have an impact on increased sexual activity.”

“This is reassuring news for teenagers, parents, and members of the public. Our study adds to growing evidence that the HPV vaccine is a safe and effective way to prevent these rare but sometimes deadly cancers,” added Robert Davis, MD, MPH, a co-author and senior investigator with Kaiser.

The study included nearly 1,400 eleven-year-old girls who were members of the Kaiser Permanente health plan in Georgia in 2006 and 2007 during the first 18 months after Gardasil became available. Of the group, 493 girls received at least one dose of the vaccine during the study period. A comparison group including 905 girls did not receive Gardasil, but did receive other recommended vaccines.

The researchers followed both groups for up to three years to assess whether they had been tested for or diagnosed with a sexually transmitted infection (STI), had taken a pregnancy test, or had been counseled about contraceptives.

Bednarczyk and colleagues found that about 10 percent of the girls in the study (counting those from both groups) had one or more of these outcomes. Only eight of the girls (less than 1 percent) were diagnosed with an STI or had a positive pregnancy test. And girls who received Gardasil did not have a substantially higher rate of testing, diagnosis, or counseling compared to those who did not receive the vaccine.

Some parents have been concerned that the vaccine was “a license to have sex,” but this latest study strengthens the evidence against that concern, Dr. Elizabeth Alderman, an adolescent medicine specialist at The Children’s Hospital at Montefiore in New York City who was not involved in the study, told CBS News.

A CDC study published in January suggested that the vaccine didn´t promote sexual activity among older girls, but it relied on self-reporting of girls from ages 15 to 24, Alderman noted, adding that self-reporting is notoriously unreliable.

Saad Omer, an infectious diseases and vaccine researcher from Emory University in Atlanta who worked on the study, told Genevra Pittman of Reuters Health: “Parents can be reassured at least based on the evidence that young girls who receive HPV vaccines did not show increased signs (of) clinical outcomes of sexual activity.”

Alderman has been a paid speaker for Merck & Co., which makes one of the two vaccines for HPV sold in the US, but stated she has no current financial ties to the company. Three of the study’s four co-authors have also reported they have done previous research funded by Merck.

“The administrative data used for this study did not provide an opportunity to do a detailed examination of the reasons for this counseling or of the extent of hormonal contraceptive use among girls in this cohort,” wrote the authors.